Modafinil (C15H15NO2S) of formula (A), also known as 2-(benzhydrylsulphinyl) acetamide or 2-[(diphenylmethyl)sulphinyl]acetamide, is a synthetic acetamide derivative with wake promoting activity, the structure and synthesis of which has been described in U.S. Pat. No. 4,177,290.

Modafinil has a stereogenic center at the sulphur atom and thus exists as two optical isomers, i.e. enantiomers.
Modafinil in its racemic form has been approved by the United States Food and Drug Administration for use in the treatment of excessive daytime sleepiness associated with narcolepsy.
U.S. Pat. No. 4,927,855 is related to modafinil enantiomers and particularly to the levorotary isomer and its use to treat depression and disorders present in patients suffering from Alzheimer disease.
According to this document, these enantiomers of modafinil are obtained by a process involving a chiral resolution method, which implies salt formation of the racemate of modafinic acid, also called benzhydrylsulphinyl acetic acid, with (−)-α-methylbenzylamine, a chiral, optically pure amine. The diastereoisomers obtained are then separated and finally one of the separated diastereoisomers is converted into the optically pure modafinic acid in a hydrolytic, or bond cleavage. The levorotary isomer of modafinic acid is thus obtained with very poor yields of about 21% from racemic modafinic acid.
Subsequently, the isolated enantiomer of modafinic acid has to be further processed by esterification and amidation steps, before the single enantiomer of modafinil can be obtained.
Thus, the modafinil enantiomer is obtained with a yield of about 6% from racemic modafinic acid, calculated on the basis of the yield of each step.
Considering alternative ways of obtaining enantiomerically pure modafinil, various metal-catalyzed enantioselective oxidations or stoichiometric transition-metal-promoted asymmetric reactions were described in the literature to prepare chiral sulphoxides by chemical oxidation of the corresponding sulphides (Kagan H. B. In “Catalytic Asymmetric Synthesis”; Ojima I., Ed. VCH: New York 1993, 203-226; Madesclaire M., Tetrahedron 1986; 42, 5459-5495; Procter D. J., Chem. Soc. PerkinTrans 1999; 835-872; Fernandez I. et al., Chem. Review 2002; A-BC). Metal-catalyzed enantioselective oxidations involve a metal catalyst complexed with a chiral ligand such as diethyl tartrate, C2-symmetric diols or C3-symmetric chiral trialkanolamine titanium(IV) complexes, C3-symmetric trialkanolamine zirconium (IV) complex, chiral (salen) manganese (III) complex, chiral (salen) vanadium (IV) complex in the presence of various oxidants such as H2O2, tert-butyl hydroperoxide, cumene hydroperoxide. Methods based on chiral oxaziridines have also been used in the chemical oxidation of sulphides.
Some enzymatic methods for the asymmetric synthesis of fine chemicals were described in Kaber K. in “Biotransformations in Organic Chemistry”, Springer Ed. 3rd ed. 1997 and reviewed by Fernandez I. et al. (Chem. Review 2002, A-BC). As an example, thioethers can be asymmetrically oxidized both by bacteria [e.g. Corynebacterium equi (Ohta H. et al. Agrig. Biol. Chem. 1985; 49:2229), Rhodococcus equi (Ohta H. et al. Chem. Lett. 1989; 625)] and fungi [Helminthosporium sp., Mortieralla isabellina sp. (Holland H L. et al. Bioorg. Chem. 1983; 12:1)]. A large variety of aryl alkyl thioethers were oxidized to yield sulphoxides with good to excellent optical purity [(Ohta H. et al. Agrig. Biol. Chem. 1985; 49:671; Abushanab E. et al., Tetrahedron Lett. 1978; 19:3415; Holland H L. et al. Can. J. Chem. 1985; 63:1118)]. Mono-oxigenases and peroxidases are important class of enzymes able to catalyse the oxidation of a variety of sulphides into sulphoxides (Colonna S. et al. Tetrahedron: Asymmetry 1993; 4:1981). The stereochemical outcome of the enzymatic reactions has been shown to be highly dependant on the sulphide structure.
As an other alternative of the enzymatic approach, optically pure methyl arylsulphinylacetates with high enantiomeric excess (>98%) obtained by lipase-catalyzed resolution of the corresponding racemate were also described (Burgess K. et al. Tetrahedron Letter 1989; 30: 3633).
As an enantioselective oxidation method, an asymmetric sulphide oxidation process has been developed by Kagan and co-workers (Pitchen, P; Deshmukh, M., Dunach, E.; Kagan, H. B.; J. Am. Chem. Soc., 1984; 106, 8188-8193). In this process for asymmetric oxidation of sulphides to sulphoxides, the oxidation is performed by using tert-butyl hydroperoxide (TBHP) as oxidizing agent in the presence of one equivalent of a chiral complex obtained from Ti(OiPr)4/(+) or (−) diethyl tartrate/water in the molar ratio 1:2:1.
The general procedure for sulphide oxidation according to Kagan comprises first performing the chiral complex at room temperature in methylene chloride before adding the sulphide. Then, the oxidation reaction is effected at −20° C. in the presence of tert-butyl hydroperoxide.
The direct oxidation of a variety of sulphides, notably for arylalkyl sulphides into optically active sulphoxides, with an enantiomeric excess (ee), in the range of 80-90%, can be achieved by this method.
More specifically, Kagan and co-workers reported that sulphoxide products could be obtained with high enantioselectivity when sulphides bearing two substituents of very different size were subjected to an asymmetric oxidation. For instance, when aryl methyl sulphides were subjected to oxidation, it was possible to obtain the aryl methyl sulphoxides in an enantiomeric excess (ee) of more than 90%.
Notably, cyclopropylphenyl sulphoxide is formed with 95% ee by this method.
However, asymmetric oxidation of functionalized sulphides, notably those bearing an ester function, was found to proceed with moderate enantioselectivity under these conditions.
Thus, compounds bearing on the stereogenic center, i.e. the sulphur atom, an alkyl moiety with an ester function close to the sulphur atom, such as methylphenylthioacetate, ethylmethylthioacetate and methylmethylthiopropanoate, are reported with ee of only 63-64% (H. B. Kagan, Phosphorus and Sulphur, 1986 27, 127-132).
Similarly, oxidation of the aryl methyl sulphides with a methyl ester function in the ortho position of the aryl group yields low enantiomeric excess (60%) and yield (50%) as compared to the para substituted compound (ee 91%, yield 50%) or to the p-tolyl methyl sulphide (ee 91%, yield 90%) (Pitchen, P et al., J. Am. Chem. Soc., 1984; 106, 8188-8193).
Hence, even when the substituents on the sulphur atom differ in size, the presence of an ester function close to the sulphur atom strongly affects the enantioselectivity of the asymmetric oxidation.
These results also show that the enantioselectivity of this process highly depends on the structure and notably on the functionality of the substrate. More specifically, oxidation of sulphides bearing an ester function close to the sulphur gives little asymmetric induction.
Similarly, none of the enantioselective reactions so far reported in the literature deals with substrates bearing an acetamide or acetic acid moiety directly linked to the sulphur atom.
There have been attempts to improve the enantioselectivity by modifying some conditions for asymmetric oxidation of sulphides. For example, Kagan and co-workers (Zhao, S.; Samuel O.; Kagan, H. B., Tetrahedron 1987; 43, (21), 5135-5144) found that the enantioselectivity of oxidation could be enhanced by using cumene hydroperoxide instead of tert-butyl hydroperoxide (ee up to 96%). However, these conditions do not solve the problem of oxidation of sulphides bearing ester, amide or carboxylic acid functions close to the sulphur atom.
Thus, the applicant obtained crude (−)-modafinil with a typical enantiomeric excess of at most about 42% with the above method using the conditions described by Kagan H. B. (Organic Syntheses, John Wiley and Sons INC. ed. 1993, vol. VIII, 464-467) (refer to Example 17, comparative Example 1 below).
H. Cotton and co-workers (Tetrahedron: Asymmetry 2000; 11, 3819-3825) recently reported a synthesis of the (S)-enantiomer of omeprazole via asymmetric oxidation of the corresponding prochiral sulphide. Omeprazole, also called 5-methoxy-2-[[(4-methoxy-3,5-dimethyl-2pyridinyl)methyl]-suphinyl]-1H-benzimidazole is represented by the following formula:

The asymmetric oxidation was achieved by titanium-mediated oxidation with cumene hydroperoxide (CHP) in the presence of (S,S)-(−) diethyl tartrate [(S,S)-(−)-DET]. The titanium complex was prepared in the presence of the prochiral sulphide and/or during a prolonged time and by performing the oxidation in the presence of N,N-diisopropylethylamine. An enantioselectivity of >94% was obtained by this method, whereas the Kagan's original method gives a modest enantiomeric excess of the crude product (30%).
According to the authors, the improved enantioselectivity of this process applied to omeprazole only is probably linked to the presence of benzimidazole or imidazole group adjacent to sulphur, which steers the stereochemistry of formed sulphoxide. The authors also suggested using this kind of functionality as directing groups when synthesizing chiral sulphoxides in asymmetric synthesis.
Hence, this publication is essentially focused on omeprazole, a pro-chiral sulphide bearing substituents of approximately the same size, and including an imidazole group which is described to play an important role in the asymmetric induction.
Therefore, there is a need for an improved enantioselective process for the manufacture of optically pure modafinil as well as other structurally related sulphoxides, notably 2-(benzhydrylsulphinyl)acetic acid and 2-(benzhydrylsulphinyl) alkyl acetate which overcomes the drawbacks of the prior art and, in particular, allows high yields.